Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into Nsindependent channels, which are also referred to as spatial channels, where Ns<min{NT, NR}. Each of the Ns independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput, greater reliability, best spectral efficiency, etc.) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system supports a time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point to extract transmit eigen-beamforming gain on the forward link when multiple antennas are available at the access point. However, in a frequency division duplex (FDD) system, the forward and reverse link transmissions are on widely separated frequencies. As a result, the forward link channel and the reverse link channel may fade independently. A direct consequence is that the reverse link channel estimates do not provide instantaneous channel knowledge of the forward link. This problem is further complicated in a system with multiple transmit and multiple receive antennas, also known as MIMO.
Thus, there is a need for method of pre-coding wherein the receiver transmits beam vector information over the reverse link and then the transmitter uses this information to transmit data in the preferred direction to receiver.